Manipulating the Genome of Human Embryos:
Some Unforeseen Effects

In recent years scientists have developed powerful tools to create
specific breaks in DNA sequences. They can then either repair those
breaks or introduce new DNA into the sequence at the site of the
break. These are called genome editing techniques. Two main techniques
at present are the CRISPR-Cas9 system and zinc-finger nucleases. In
principle, researchers can modify any part of the genome. They have
achieved intended modifications in experiments with human and animal
cells and also with mouse embryos. But the specific alterations do not
always occur and there are also unintended effects.

The techniques are not as precise as they are sometimes made out to be,
so there is every reason for caution in their application, especially in
connection with the manipulation of human cells or human embryos. But
over and beyond technical issues is the pressing ethical concern:
should researchers cross the line into genetically manipulating
human embryos?

Knowing that scientists in China were performing these
experiments, two groups of researchers and others published
comments in Science and Nature in March 2015 warning
about genome editing in human embryos (Baltimore et al. 2015; Lanphier
et al. 2015). Since then, one Chinese research group has published
the results of its experiments (Liang 2015).

Using the CRISPR-Cas9 system, the group’s aim was to create a break in the
beta-globin gene (β-globin or HBB; a key hemoglobin gene) and
then repair that break. The context for the experiment is the heritable
disease beta-thalassemia (β-thalassemia, also written “β-thalassaemia”), a
blood disease that is
related to a mutation in the β-globin gene. If researchers could create a
targeted break in the mutated part of the gene and then repair it, they
could theoretically prevent the disease from occurring. The current
experiment was a first step in this direction, a test of the efficacy and
precision of the technique in human embryos.

In this experiment the researchers used 86 abnormal human embryos that
they had obtained from fertility clinics that perform in vitro
fertilization (IVF); the embryos were donated by the parents. Each of the
single-celled embryos (zygotes) contained two sperm nuclei, instead of
one sperm nucleus that normally fuses with the egg nucleus. Such abnormal
embryos do not develop further when transferred into a woman’s womb
and would have been discarded by the fertility clinics. For this reason
the researchers state that their experiment avoids any ethical issues,
since they were working with embryos that would have been thrown away
anyway. (It is a symptom of the distancing effect of technology that
it is possible to casually speak of discarding or throwing away human
embryos. In stating that their experiment is ethically unproblematic,
they do not question the ethics of IVF techniques that engender
abnormal human embryos in the first place.)

The 86 embryos were injected with the genetic construct intended to create
the break and then repair the β-globin gene. After 48 hours (at about the
8-cell stage in early embryonic development) 71 of the embryos were
viable. Fifty-four of these were tested to see whether the editing had
occurred as intended. The creation of the break succeeded in 28 embryos,
but only in four had the gene been repaired in the intended fashion. In
seven other embryos the “repair” was based on a similar gene (HBD)
in the embryo and not on the introduced construct. This indicated that the
embryo itself was active in trying to put right what had been disturbed by
the intervention. From a technical point of view, the embryo got in the
way of the manipulation, since an experiment is most successful when only
that occurs which is intended.

There were other unforeseen effects. Each of the embryos was in respect
to the edited part of the genome a “mosaic.” This means that the
sequence of nitrogenous bases in the DNA strands of the “repaired”
part of the gene varied in different cells of the same embryo. In other
words, the intended uniform and controlled editing process did not
occur. In addition, the researchers discovered mutations in two other
genes that were directly caused by the genetic manipulation. Since they
only looked at selected sites in the genome to screen for mutations, they
surmise that they “likely underestimated the off-target effects”
in the embryos.

The results show that the technique used is far from trustworthy in
producing clearly circumscribed and uniform effects. From a purely
technical perspective there can be no doubt that it would be wholly
irresponsible at this time to use the technique to genetically alter a
human embryo. There is also little doubt that scientists will continue
research in this area and improve the techniques. And some scientists
will argue that to truly improve the techniques, experiments on human
embryos will need to be performed. How could you prove the methods
are safe for humans without having tested them on humans? The urge to
do what is potentially doable, regardless of any larger context, is
not to be underestimated as a powerful driver of technology-oriented
scientific research.

At the same time as this drive motivates the development of ever finer
manipulative techniques, the broader field of genetics is increasingly
discovering how all genetic processes are highly context dependent. For
this reason, David Baltimore and 17 co-authors, including some who are
leading efforts to develop gene-editing technologies, wrote recently
that the “potential for unintended consequences of heritable germline
modifications” will remain a serious concern since “there are limits
to our knowledge of human genetics, gene-environment interactions, and
the pathways of disease (including the interplay between one disease
and other conditions or diseases in the same patient)” (Baltimore
et al. 2015).

We can take as an example the fairly common blood disorder, β-thalassemia.
As mentioned above, it is normally connected with mutations in the
β-globin gene that is involved in the formation of hemoglobin, the vital
iron-binding molecule in red blood cells. When little (or no) β-globin is
synthesized due to a mutation, then hemoglobin formation and function are
compromised, which can lead to a variety of symptoms in the affected
person. Over 200 different types of mutations in the β-globin gene or in
DNA that is related to its production have been discovered. The β-globin
gene is located in chromosome 11, of which there are two. If there is a
mutation in only one of the β-globin genes, a person usually has no
symptoms. (In genetic terms, they are called carriers.) When there are
deleterious mutations in both β-globin genes, then a person normally
has symptoms, but they can range from light anemia to the need for ongoing
blood transfusions. As a review article states,

For more than 25 years researchers have questioned why patients who are
homozygous for identical molecular defects in the β-globin genes can
have such remarkably different phenotypes. Some patients need regular
blood transfusion (β-thalassaemia major), whereas others are transfusion
independent (β-thalassaemia intermedia). (Higgs et al. 2012)

So at first one imagines a straightforward Mendelian recessive disease in
which a person has deleterious mutations in both of its β-globin genes
(is “homozygous”) and therefore has the disease. But it turns out
that there is no one-to-one correlation between β-globin mutations and
how the disease actually develops. There are many additional factors that
influence the disease, which has also been found to be the case in other
“simple” Mendelian diseases.

Some people produce no β-globin and
some produce reduced amounts, but this “does not necessarily predict
disease severity, however; people with both types have been diagnosed
with thalassemia major and thalassemia intermedia,” as the description
of the disease at the National Institutes of Health (NIH) website
states (http://ghr.nlm.nih.gov/condition/beta-thalassemia). Carriers
who have β-globin mutation in only one of the two β-globin genes
are normally healthy and, interestingly, have been found to be
“relatively protected” against malaria (Cao and Galanello 2010),
a fact that calls into question the idea of totally eradicating
β-globin mutations. However, carriers sometimes also have a mild form of
the disease β-thalassemia.

That a seemingly simple genetically conditioned
disease is turning out to be highly complex is brought home by a study
of β-thalassemia in China that examined 117 individuals with a mild
form of the disease (β-thalassemia intermedia). They found, as have
other studies, “a high degree of heterogeneity in both phenotypic
and genotypic aspects” (Chen et al. 2010). Most surprising was the
discovery that among the patients were three individuals who produced
no β-globin at all and nonetheless had only mild anemia requiring
occasional transfusions. The bodies of some individuals evidently have
possibilities of compensating for the lack of β-globin, while in
other individuals still other factors play a role leading to a worse
condition than a genetic analysis would indicate. Researchers are finding
an increasing variety of such “genetic modifiers.”

So what is
in any case clear is the gulf that yawns between genetic diagnosis and
the actual disease. Increasing knowledge about the genetic factors will
not necessarily help patients who are suffering from the disease. What
might a couple learn, for example, from a genetic diagnosis they choose
to have performed because both individuals have family histories in
which β-thalassemia occurred? For example, if both are diagnosed as carriers
of one β-globin mutation and want to have a child, then there is a 25%
likelihood that the child will have a β-globin mutation in both of
its β-globin genes. They could have the embryo (or later the fetus)
genetically screened to see if it does in fact carry both mutant
genes. If it does, then the parents know that the child will likely
have some form of the disease.

But it does not tell the parents about how this genetic condition will
actually affect their particular child—will it be mild, will there be
serious complications, will the child need frequent transfusions? The
parents now have some abstract information and hopefully a clear sense of
how little they actually can know on the basis of the genetic diagnosis.
They can choose to have the child or they may decide to have an abortion.
That is where we stand today. As genetic diagnostic techniques are further
refined, couples will have more and more information. But will it really
tell them much, or mainly increase their feeling of facing uncertainty?

And what if one could “repair” the mutated part of the β-globin gene
right at the beginning of conception? That is the goal that stands behind
the genome-editing experiment performed by the Chinese scientists. The
parents would have to agree to in vitro fertilization so that
the gene-editing process would take effect before the zygote begins
to divide into a multicellular organism. But at this stage you cannot
know whether that particular zygote actually has no, one or two mutant
β-globin genes. There is a 75% chance that the manipulation would be
performed on an embryo that will not develop the disease (that has no
mutant β-globin genes or only one as a “carrier”). In other words,
the scientists/physicians would most likely be manipulating a completely
healthy zygote and have no way of knowing what side-effects (which could
be heritable) the manipulation would have. As geneticist Rudolph Jaenisch
states, “it is unacceptable to mutate normal embryos. For me, that
means there is no application [of this technique in human embryos]”
(quoted in Kolata 2015).

What stands behind Jaenisch’s statement is the foundational conviction
that the medical profession exists to help patients who are suffering. A
physician does not make an intervention in a healthy person that may
cause illness. What could be clearer?

That scientists are nonetheless pursuing research that would lead to doing
just this is as remarkable as it is disturbing. It shows how disconnected
the seemingly logical or “elegant” idea “correct a gene, prevent a
disease” is from biological reality. They may truly have the best
intentions and they may also be seduced by the Sirens of technology that
invite us to pursue the pathway of the technologically doable for its own
sake. When we accept this invitation, we are guided by a
de-contextualized idea of a disease and the seductive promise of a
technique viewed in isolation from concrete life.

The history of technology shows that society has often allowed technology
to take on a life of its own, only to be confronted later
with all the unintended consequences that had often been foreseen.
Will we be any wiser when it comes to manipulating human embryos?

Craig Holdrege, Ph.D., is the director of The Nature Institute in Ghent,
NY. The Institute works through education, research, and publications to
inspire a new paradigm for science and technology — a paradigm that
encourages us to strive for a healthy future by embracing nature’s wisdom.
Craig is also author of Thinking Like a Plant: A Living Science for
Life (Lindisfarne Books, 2013) and co-author of Beyond
Biotechnology: The Barren Promise of Genetic Engineering (University
Press of Kentucky, 2008). He can be reached at craig@natureinstitute.org.